It is going to take a little longer than expected to get a so-called "theory of everything'' to explain the Universe, says Stephen Hawking.

The world famous physicist is attending a conference on string theory, the idea that on a fundamental level all matter and indeed space and time itself is made of tiny loops of energy.

Hawking: Trying to answer an age-old question

In the 1980s, when string theory was fairly new, he said there was a 50-50 chance the theory would be proven in 20 years. Now he is not so sure.

"Although we have made great progress in the last 20 years, we don't seem much nearer to our goal,'' he said. He now believes it could take another 20 years.

Some scientists hope that string theory could unite the two main theories of physics: Einstein's theory of general relativity which deals with the large scale structure of space and time, and quantum theory which deals with the fundamental nature of matter and energy.

Mutually incompatible

The problem is that while both theories are spectacularly good at explaining the Universe within their own terms, they are mutually incompatible. They cannot both be right.

In fact, probably neither them is. They may be part of a larger more profound theory. String theory is one candidate, by no means the only one, for such a "theory of everything."

The weeklong annual conference he is attending is called Strings '99. It is being held in Germany for the first time. It takes place close to Albert Einstein's summer home, where he spent his last years in Germany pondering a unifying theory for physics.

After he left Germany, he continued work on a so-called unified theory at Princeton University, but never found a solution. According to Hawking, however, a unified theory may not have a solution that works all the time.

"There may be no theory that can be applied in different situations," he said, speaking through a computer attached to his wheelchair," just as there is no map that covers the entire world.

"We don't understand the origin of the Universe or why we are here,'' he added. "A complete unified theory might not bring much material benefit, but it would answer that age-old question.''